1. Field of the Invention
The present invention relates to a starter for cranking an internal combustion engine, the starter having a planetary gear speed reduction device that reduces a rotational speed of an electric motor, and more particularly to an excessive-torque-absorbing device used together with the planetary gear speed reduction device.
2. Description of Related Art
Examples of an excessive-torque-absorbing device used in a starter are disclosed in JP-A-63-277859 and JP-A-11-117946. The excessive-torque-absorbing device disclosed therein is composed of a rotatable disk connected to an internal gear, a pair of fixed disks sandwiching the rotatable disk from both sides and a disk spring that presses the fixed disks in the axial direction to thereby generate a frictional force between the rotatable disk and the fixed disks. When an excessive rotational torque is applied to the rotatable disk from a pinion gear abutting a ring gear of an internal combustion engine, the rotatable disk slips relative to the fixed disks, and thereby the internal gear connected to the rotational disk rotates. Thus, the excessive torque is absorbed in the device. In another example shown in U.S. Pat. No. 6,076,413, the rotatable disk itself forms the internal gear, and the rotatable disk is pressed from both sides by a pair of fixed disks in the same manner as in the former examples.
In the conventional device disclosed in those documents, only a single rotabable disk that is sandwiched between a pair of fixed disks is used. Therefore, it is difficult to obtain a high gripping force between the rotatable disk and the fixed disks. On the other hand, a high gripping force is required for transmitting a high rotational torque for cranking a diesel engine, for example. If the rotatable disk is pressed too hard by the fixed disks to obtain a high gripping force, friction surfaces of the disks would be damaged by heat generated by friction, or seizing between disks would occur, because the frictional surfaces are not wide enough.
In particular, in the device disclosed in U.S. Pat. No. 6,076,413, the friction surface of the rotatable disk is located outside of the internal gear teeth. Therefore, the width of the friction surface is limited and the friction area cannot be made wide enough. If the force pressing the rotatable disk from both sides is increased to obtain a high gripping force, the friction surface would be damaged soon. It would be possible to increase the gripping force by increasing a friction coefficient of the friction surface. In this case, however, the high gripping force has to be received by the fixed disks having insufficient friction surface. To avoid damages in the fixed disks, the fixed disks have to be made thick and a structure supporting the fixed disks has to be strengthened. If the fixed disks are made thick, seizing between the disks would be caused more easily when the fixed disks are distorted for some reasons in a manner to reduce the friction area. If the supporting structure is strengthened, it is unavoidable to make the starter bulky.